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  fs<span class="token punctuation">.</span><span class="token function">readFile</span><span class="token punctuation">(</span>filename<span class="token punctuation">,</span> <span class="token punctuation">(</span><span class="token parameter">err<span class="token punctuation">,</span> data</span><span class="token punctuation">)</span> <span class="token operator">=&gt;</span> <span class="token punctuation">{</span>
    <span class="token keyword">if</span><span class="token punctuation">(</span>err<span class="token punctuation">)</span> <span class="token punctuation">{</span>
      <span class="token function">reject</span><span class="token punctuation">(</span>err<span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token punctuation">}</span><span class="token keyword">else</span> <span class="token punctuation">{</span>
      <span class="token function">resolve</span><span class="token punctuation">(</span>data<span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token punctuation">}</span>
  <span class="token punctuation">}</span><span class="token punctuation">)</span>
<span class="token punctuation">}</span>
<span class="token function">readFilePromise</span><span class="token punctuation">(</span><span class="token string">'1.json'</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">then</span><span class="token punctuation">(</span><span class="token parameter">data</span> <span class="token operator">=&gt;</span> <span class="token punctuation">{</span>
  <span class="token keyword">return</span> <span class="token function">readFilePromise</span><span class="token punctuation">(</span><span class="token string">'2.json'</span><span class="token punctuation">)</span>
<span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
</code></pre></div><p>看到没有，回调函数不是直接声明的，而是在通过后面的 then 方法传入的，即延迟传入。这就是回调<code>函数延迟绑定</code>。</p> <p>然后我们做以下微调:</p> <div class="language-js extra-class"><pre class="language-js"><code><span class="token keyword">let</span> x <span class="token operator">=</span> <span class="token function">readFilePromise</span><span class="token punctuation">(</span><span class="token string">'1.json'</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">then</span><span class="token punctuation">(</span><span class="token parameter">data</span> <span class="token operator">=&gt;</span> <span class="token punctuation">{</span>
  <span class="token keyword">return</span> <span class="token function">readFilePromise</span><span class="token punctuation">(</span><span class="token string">'2.json'</span><span class="token punctuation">)</span><span class="token comment">//这是返回的Promise</span>
<span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
x<span class="token punctuation">.</span><span class="token function">then</span><span class="token punctuation">(</span><span class="token comment">/* 内部逻辑省略 */</span><span class="token punctuation">)</span>
</code></pre></div><p>我们会根据 then 中回调函数的传入值创建不同类型的Promise, 然后把返回的 Promise 穿透到外层, 以供后续的调用。这里的 x 指的就是内部返回的 Promise，然后在 x 后面可以依次完成链式调用。</p> <p>这便是<code>返回值穿透</code>的效果。</p> <p>这两种技术一起作用便可以将深层的嵌套回调写成下面的形式:</p> <div class="language-js extra-class"><pre class="language-js"><code><span class="token function">readFilePromise</span><span class="token punctuation">(</span><span class="token string">'1.json'</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">then</span><span class="token punctuation">(</span><span class="token parameter">data</span> <span class="token operator">=&gt;</span> <span class="token punctuation">{</span>
    <span class="token keyword">return</span> <span class="token function">readFilePromise</span><span class="token punctuation">(</span><span class="token string">'2.json'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">then</span><span class="token punctuation">(</span><span class="token parameter">data</span> <span class="token operator">=&gt;</span> <span class="token punctuation">{</span>
    <span class="token keyword">return</span> <span class="token function">readFilePromise</span><span class="token punctuation">(</span><span class="token string">'3.json'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">then</span><span class="token punctuation">(</span><span class="token parameter">data</span> <span class="token operator">=&gt;</span> <span class="token punctuation">{</span>
    <span class="token keyword">return</span> <span class="token function">readFilePromise</span><span class="token punctuation">(</span><span class="token string">'4.json'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
</code></pre></div><p>这样就显得清爽了许多，更重要的是，它更符合人的线性思维模式，开发体验也更好。</p> <p>两种技术结合产生了链式调用的效果。</p> <p>这解决的是多层嵌套的问题，那另一个问题，即每次任务执行结束后分别处理成功和失败的情况怎么解决的呢？</p> <p>Promise 采用了<code>错误冒泡</code>的方式。其实很简单理解，我们来看看效果:</p> <div class="language-js extra-class"><pre class="language-js"><code><span class="token function">readFilePromise</span><span class="token punctuation">(</span><span class="token string">'1.json'</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">then</span><span class="token punctuation">(</span><span class="token parameter">data</span> <span class="token operator">=&gt;</span> <span class="token punctuation">{</span>
    <span class="token keyword">return</span> <span class="token function">readFilePromise</span><span class="token punctuation">(</span><span class="token string">'2.json'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">then</span><span class="token punctuation">(</span><span class="token parameter">data</span> <span class="token operator">=&gt;</span> <span class="token punctuation">{</span>
    <span class="token keyword">return</span> <span class="token function">readFilePromise</span><span class="token punctuation">(</span><span class="token string">'3.json'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">then</span><span class="token punctuation">(</span><span class="token parameter">data</span> <span class="token operator">=&gt;</span> <span class="token punctuation">{</span>
    <span class="token keyword">return</span> <span class="token function">readFilePromise</span><span class="token punctuation">(</span><span class="token string">'4.json'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">catch</span><span class="token punctuation">(</span><span class="token parameter">err</span> <span class="token operator">=&gt;</span> <span class="token punctuation">{</span>
  <span class="token comment">// xxx</span>
<span class="token punctuation">}</span><span class="token punctuation">)</span>
</code></pre></div><p>这样前面产生的错误会一直向后传递，被 catch 接收到，就不用频繁地检查错误了。</p> <h2 id="解决效果"><a href="#解决效果" class="header-anchor">#</a> 解决效果</h2> <ol><li>实现链式调用，解决多层嵌套问题</li> <li>实现错误冒泡后一站式处理，解决每次任务中判断错误、增加代码混乱度的问题</li></ol></div> <footer class="page-edit"><!----> <!----></footer> <div class="page-nav"><p class="inner"><span class="prev">
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